This invention relates to thermal ink jet printheads and more particularly to thermal ink jet printheads having optimized continuous tone to high resolution text printing performance through control of image area coverage.
In one approach to continuous tone and/or grey scale printing, a pixel location may be printed with one to seven droplets, thus providing eight grey scale levels. This requires repeated use of the printhead heating elements to eject ink droplets from the printhead nozzles, thus decreasing the heating element life times and imposing a reduced printing rate. In another approach to continuous tone printing, as disclosed, for example, by U.S. Pat. 4,353,079 to Kawanabe, multiple ink droplet generators simultaneously eject droplets in different numbers to achieve different corresponding ink droplet volumes at the same pixel locations. This type of grey scale printer requires that the nozzles be critically aligned with respect to each other, so that the ink droplets will properly register within the pixel location on the recording medium.
U.S. Pat. No. 4,746,935 to Allen discloses a thermal ink jet printer having three binary weighted drop generators which are fired in sequence to produce an eight-level halftone printing process. One, two, or all three drop generators sequentially eject droplets of varying volume to the same pixel location as the drop generators are scanned across a recording medium. For multicolor printing, each ink color has a separate series of three binary weighted drop generators.
U.S. Pat. No. 5,059,989 to Eldridge et al. discloses a thermal ink jet printer having its heating elements on the edge of a substrate with its addressing electrodes and common return on opposing surfaces of the substrate. A second substrate with a recess which opens at one edge provides the ink reservoir, and a nozzle plate covers the edges. The nozzles in the nozzle plate are aligned with the heating elements and have recesses to direct the ink to the nozzles and provide ink flow barriers to prevent cross talk. For higher resolution printing, two printhead areas are combined with their nozzles staggered.
U.S. Pat. No. 3,977,007 to Berry et al. discloses shades of gray produced by an ink jet printer by depositing a predetermined number of drops at each dot or pixel location within a matrix cell. The number of drops of ink producing the desired shade is based upon the location of a dot within the matrix cell in which the number of drops are selectively adjusted by one. The desired darkness or tonal density of each dot in the cell is determined independently of every other dot in the cell. In this way, contrast can be maintained even if a white-black transition occurs in the middle of a cell.
U.S. Pat. No. 5,016, 191 to Radochonski discloses a pixel processor which converts the line descriptions from the main processor into a bit map for a half tone picture. The pixel processor initially stores input data from the main processor indicating intensity threshold levels for each pixel of a half tone cell. When processing each line, the pixel processor addresses and reads a succession of pixel data words out of the bit map, each pixel data word including at least one bit corresponding to a pixel along the path of the line. For each such bit, the pixel processor determines the half tone cell position of the corresponding pixel, determines whether the intensity threshold level assigned to that halftone cell position is lower than the intensity level of the line and sets the state of the bit accordingly. After suitable altering relevant bits of each pixel data word, the pixel processor writes the altered pixel data word back into the bit map memory at the same address.
U.S. Pat. No. 4,280,144 to Bacon discloses an apparatus and method for improving the print quality of a coarse scan but fine print image processing device. A coarsely scanned pixel is assigned a grey scale code. The assigned code indicates the reflectance characteristics of the pixel. For fine reproduction of coarsely scanned data, the coarsely scanned pixel is summed with at least four adjacent horizontal and vertical pixels to reproduce a fine pixel comprising a cell of at least four sub-elements or printable pixels from a reproducing device, such as an ink jet printer.
U.S. Pat. No. 5,012,257 to Love et al. discloses a color ink jet printing system wherein each pixel of graphics data is processed to form a 2 by 2 array of cells, each cell corresponding to a pixel area on a recording medium. A 2 by 2 array of cells is referred to as a super pixel, and the graphics data is processed to form a superpixel for each color, indicating cell location and color of ink droplet to be applied to each cell. The superpixels are controlled so that ink droplets are deposited only in a diagonally adjacent pair of cells with no more than two ink droplets per cell and no more than three ink droplets per superpixel, thereby providing printed images having the desired color and color saturation, while minimizing bleed across color field boundaries.
U.S. Pat. No. 4,999,646 to Trask discloses a multiple pass complementary dot pattern ink jet printing process. Using this process, successive printed dots of adjacent rows are offset from each other, and successive printed swaths are made by depositing first and second partially overlapping complementary dot patterns on a recording medium. Thus, the dot spacing in coincident dot rows within the overlapping portions of the dot patterns is alternated between dots in the first pattern and dots in the second pattern.
U.S. Pat. No. 4,412,226 to Yoshida discloses a plurality of ink jet printheads to print arrays of cells, each cell in the array being printable with a variable size ink droplet to produce half tone images.